The present invention relates to a method of injection molding an improved multi-layer preform for use in blow molding a container such as a beverage bottle having a bottom with improved structural strength and durability.
More specifically, the present invention relates to a method of injection molding a multi-layer preform from a combination of virgin and recycled plastic such as PET so that the resultant blow molded bottle has a reduced tendency for bottom failure in spite of the use of the recycled plastic.
It is desirable to be able to fabricate multi-layer plastic beverage bottles from the combination of virgin and recycled plastics, such as PET. Ideally recycled PET comprises a central layer of a three-layer bottle structure wherein the inner and outer layers both comprise a higher grade of more expensive virgin PET, so that neither the beverage in the bottle nor the consumer comes into direct contact with the recycled plastic (PET).
Typically the quality and strength of plastic is measured by its intrinsic viscosity (IV). For example, blow molded plastic PET bottles are generally fabricated from virgin PET having an IV from about 0.70 to about 0.86 dl/g. On the other hand recycled PET generally has an IV less than 0.74 and from about 0.63 to 0.72.
The lower intrinsic viscosity, recycled PET, is less durable than the higher IV, virgin PET. Therefore, it is subject to a higher probability of creep, stress cracking, impact failure, burst failure or cracking from various other causes. This problem is particularly acute in the bottom region of the bottle which is subject to more wear and tear.
In stretch blow molding plastic containers, a preform is typically injection molded and thereafter internally pressurized within a blow mold in order to form a bottle. The sidewalls or body of the preform are materially expanded during the blow molding process causing biaxial orientation of the plastic and increase strength. However, the bottom region of the preform is not materially expanded to the same extent and, therefore, does not undergo biaxial orientation to any significant degree that will result in increased strength. Therefore, regardless of the type of material utilized, the bottom region of the bottle is inherently weaker than the sidewall regions forming the body.
Accordingly, a need in the art exists for a method to minimize the quantity or effect of the use of recycled plastics such as PET in the bottom of a multi-layer beverage bottle since this bottom region is already inherently less durable due to the lack of biaxial orientation from the stretch blow molding process.